With open wounds, intensive swelling and lack of blood flow to the arm, the chances of Metz making it out of his house alive were very slim. He had a perfect storm of symptoms, and time was running out as he decided on self-amputation.

"When the blood flow is cut off to the tissues, the arm is basically dead after six hours," said Dr. Jason Ganz, plastic surgeon and hand surgeon at Stony Brook University Medical Center. "And this man was trapped for 18 hours in this situation. With the swelling further increasing the size of the arm in that narrow confined space, there would have been no blood flow."

Metz sustained simultaneous multiple traumas to his arm after it got ensnared in his furnace's vents as he tried to retrieve a fallen vacuum cleaner piece. When he eventually began to smell his own flesh rotting, he knew he had to go to extreme measures, he told CNN.

"It took me about six hours to psych myself up to the point where I thought I was capable of actually doing what I thought needed to be done," Metz told CNN.

If Metz's injury had occurred in a cold climate, the outcome might have been very different. The prompt reattachment of an amputated limb that has been preserved on ice can be successful.

But in Metz's case, the area where he was trapped wasn't cold enough to preserve the arm, and it was most likely dirty, as most boiler rooms are. Thus the lacerations on his arm, coupled with the poor conditions, established a high potential for infection.

And the swelling in a vulnerable area of the arm increased the possibility for disastrous consequences.

If he'd trapped a hand, it might have been a different story. "The hand has more bone and tendon and less muscle so you have a little more time, maybe eight hours, before the tissue dies," Ganz said. "But the area of the arm above the elbow that is closer to the shoulder has a lot of muscle, and thus a higher demand for blood and nutrients."

"He did enough to isolate the toxins and to protect himself from the systemic effects of a gangrenous limb," Herman said. "It's my understanding that he could not complete the amputation because he had gotten close to the nerves and it became too painful. But he was able to prevent a systemic blood infection, and to avoid the poisons that are liberated when parts of the body start to die."